rice grass vs cereal grass

rice grass and cereal grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
rice grass	Yes	No	No	No
cereal grass	Yes	No	No	No

As nouns, cereal grass is a hypernym of rice grass; that is, cereal grass is a word with a broader meaning than rice grass:

rice grass: any grass of the genus Oryzopsis
cereal grass: grass whose starchy grains are used as food: wheat, rice, rye, oats, maize, buckwheat, millet

Other hypernyms of rice grass include cereal.

rice grass (noun)	cereal grass (noun)
any grass of the genus Oryzopsis	grass whose starchy grains are used as food: wheat, rice, rye, oats, maize, buckwheat, millet